Display device with temperature stabilization

ABSTRACT

The present invention relates to an image display system comprising a display panel ( 1 ) having at least one channel ( 35, 37, 39 ) comprising an ionizable gas mixture ( 55 ) including at least a basic gas and an additional gas. The walls of the channel are provided with electrodes ( 41, 43 ) for selectively ionizing the ionizable gas mixture during operation. The display system further comprises means for supplying additional gas to the ionizable gas mixture. The additional gas is a gas of the group formed by deuterium, hydrogen and deuterium hydrogen. The image display system is provided with temperature-stabilizing means for stabilizing the temperature of the means for supplying deuterium, hydrogen or deuterium hydrogen.

BACKGROUND OF THE INVENTION

The invention relates to a display device having a display panelcomprising at least one compartment which contains an ionizable gasmixture including at least a basic gas and an additional gas, thecompartment being provided with electrodes for selectively ionizing theionizable gas mixture during operation, and the display panel comprisingmeans for supplying the additional gas to the ionizable gas mixture.

Display devices for displaying monochromatic images or color imagescomprise plasma-addressed liquid crystal display devices, referred to asPALC displays, preferably of the flat-panel type. PALC displays areused, for example, as displays for television and computer applications.

A display device of the type described in the opening paragraph is knownfrom, for example published European patent application EP 0 816 898.The flat-panel type display device described in this application has adisplay screen with a pattern of (identical) data storage or displayelements and a plurality of compartments. The compartments are filledwith an ionizable gas mixture and are provided with electrodes forselectively ionizing the ionizable gas mixture during operation. In theknown display device, the compartments have the shape of parallel,elongated channels in the form of a channel plate functioning asselection means for the display device. These are the plasma-addressedrow electrodes. By applying a voltage difference across the electrodesin one of the channels of the channel plate, electrons are emitted fromthe cathode. The electrons will ionize the ionizable gas so that aplasma is formed. When the voltage across the electrodes in one channelis switched off and the gas mixture is deionized, a subsequent channelis switched on. At the display screen side of the display device, thecompartments are closed by a relatively thin dielectric layer referredto as the microsheet. This layer is provided with electro-opticalmaterial, and further electrodes which function as the data electrodesor column electrodes of the display device. The further electrodes areprovided on a substrate. The display panel is constituted by theassembly of the channel plate with the electrodes and the ionizable gasmixture, the dielectric layer, the layer of the electro-optical materialand the further electrodes.

In a PALC display panel, the panel is addressed row by row. Theresolution is determined by the number of rows which can be written persecond. The minimally required time for writing a row is determined bythe time required to ignite the plasma, the time required to charge themicrosheet and the time required to switch off the plasma, referred toas the afterglow decay time. The shorter the afterglow decay time, thelarger the number of rows which can be written per second and the higherthe maximally achievable resolution.

A gas mixture which is known to be suitable for a relatively shortafterglow decay time is a He-H₂ mixture. However, a problem with such amixture is the stability with respect to time. The quantity of H₂ isrelatively small (typically less than 3%) and the probability that theequilibrium pressure of H₂ in the mixture remains constant is verysmall. This pressure variation is caused by the loss of hydrogen, forexample, due to implantation in the electrodes during operation of thepanel, or because there may be hydride formation. At a too low hydrogenpressure, the afterglow decay time will be too long, while the plasmawill no longer function optimally at a too high hydrogen pressure. SaidEuropean patent application proposes providing the panel with a materialwhich supplies and absorbs hydrogen so as to control the hydrogenpressure in the compartments.

A drawback of this solution is that, for the material used as a hydrogensource, the equilibrium pressure of hydrogen in the material isdependent on the temperature. Since the operating temperature of a PALCdisplay panel may vary between 0° C. and about 80° C., a variation ofmore than a factor of 100 may occur in the hydrogen pressure, which isnot acceptable.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a display device inwhich the hydrogen pressure in the display panel is maintainedsubstantially constant.

To this end, the display device according to the invention ischaracterized in that the display device is provided withtemperature-stabilizing means for stabilizing the temperature of themeans for supplying the additional gas.

Since there is a loss of the additional gas, resulting in a reduction ofthe concentration of the additional gas, and since the afterglow decaytime preferably remains below a given value, it is desirable to takemeasures so as to maintain the partial pressure of the additional gas inthe ionizable gas mixture substantially constant. The partial pressureof the additional gas can be controlled by providing the display devicewith means which supply additional gas. The materials used for thispurpose yield a constant partial pressure of the additional gas within agiven concentration range of the additional gas. For this givenconcentration range, this pressure is thus substantially independent ofthe concentration of the gas in the material, but is dependent on thetemperature of the material. By stabilizing the temperature of thematerial supplying additional gas, said temperature dependence and,consequently, the variation of the partial pressure of the additionalgas in the ionizable gas mixture is compensated. In this way, thepressure of the additional gas in the compartments can be maintained atthe desired level.

A preferred embodiment of the display device according to the inventionis characterized in that the additional gas is a gas of the groupconstituted by hydrogen, deuterium, or hydrogen deuterium.

Important parameters of the plasma discharge cycle of the display deviceare the electric conductivity of the plasma discharge and the decreaseof conductivity in the afterglow decay time. When the decrease of theconductivity of the plasma discharge progresses too slowly, thedischarge may continue while a subsequent data row is already beingwritten, which is undesirable. A conductivity which decreases toorapidly also has detrimental effects.

Helium (He) is the most frequently used basic gas in display devices ofthe type described in the opening paragraph. The ignition voltage of theplasma discharge may be decreased by adding small quantities of a gas(typically of the order of several percents) to the helium. Such gasesgenerally have an ionization potential which is lower than that ofhelium. The gas mixtures formed are referred to as Penning mixtures. Aknown additional gas is hydrogen (H₂). By using such gas mixtures, notonly the ignition characteristic of the plasma discharge is influencedbut also, for example, the current which is necessary to maintain thedischarge, and the afterglow decay characteristic of the discharge. Theproperties of the plasma discharge cycle of the display device can beinfluenced by making a suitable choice of the additional gas. Theadditional gas may be alternatively deuterium (D₂) or hydrogen deuterium(HD), so that, in comparison with the addition of hydrogen, the plasmadischarge will have a lower ignition and sustain voltage and a longerafterglow decay time.

A further embodiment of the display device according to the invention ischaracterized in that the temperature-stabilizing means comprise aPeltier element accommodating the means for supplying the additionalgas.

A Peltier element is a very suitable element with which the means forsupplying the additional gas can be both cooled and heated.

A further embodiment of the display device according to the invention,comprising an exhaust tube, is characterized in that the Peltier elementis accommodated in the exhaust tube, and in that at least a part of theheat exchanger of the Peltier element forms part of the wall of theexhaust tube.

The exhaust tube has an exhaust connection for the display device. Theheat exchanger forms part of the wall of the exhaust tube in order tosupply heat to the ambiance or to a heat sink.

Another embodiment of the display device according to the invention ischaracterized in that the temperature-stabilizing means comprise a wireon which the means for supplying the additional gas are provided, thetemperature of said wire being variable.

By passing a current through the wire, the wire will be heated. Sincethe resistance of the wire is dependent on the temperature, thetemperature of the wire can be measured and controlled by measuring theresistance.

A further embodiment of the display device according to the invention,comprising an exhaust tube, is characterized in that the wire isaccommodated in the exhaust tube.

A further embodiment of the display device according to the invention ischaracterized in that the means for supplying the additional gascomprise a material from the group constituted by VZ₂, LZ₃, PdZ_(0.6),LaNi₅Z₆, LaNi₂Z_(x), LaCo₅Z_(x), Zr-Mn-Z_(x) and Pd-Ag-Z_(x), in which Lis a lanthanide and Z is hydrogen and/or deuterium.

The above-mentioned materials are particularly suitable as means forsupplying hydrogen, deuterium or hydrogen deuterium. Consequently, theyare hydride-forming materials which have a plateau for a givenconcentration range in the desired pressure range. All of thesematerials have a relatively flat part in a relatively largeconcentration range of additional gas in the guest material. Thus, thesematerials supply a constant partial pressure of the additional gas,which is substantially independent of the concentration of the gas inthe guest material and its temperature dependence is compensated in thepresent invention.

The temperature-stabilizing means mentioned so far are providedinternally in the display panel. Another embodiment of the displaydevice according to the invention, comprising a background illuminationunit, is characterized in that the background illumination unit formspart of the temperature-stabilizing means.

An alternative embodiment of the display device according to theinvention, comprising an exhaust tube, is characterized in that aheating element surrounds the exhaust tube.

The temperature stabilization in this case is also effected by meanswhich are present within the display device but outside the displaypanel. This solution is relatively simple. The heating element may be,for example a helical heating wire.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a block diagram of a display panel;

FIG. 2 shows diagrammatically a part of a plasma-addressed liquidcrystal display device in a perspective view;

FIG. 3 shows the pressure of hydrogen as a function of the hydrogenconcentration in the guest material;

FIG. 4 shows diagrammatically a part of a plasma-addressed liquidcrystal display device in a perspective view;

FIG. 5 shows the integration of a Peltier element with the guestmaterial in the exhaust tube;

FIG. 6 shows the integration of a wire provided with the guest materialin the exhaust tube; and

FIG. 7 shows the exhaust tube provided with an external heating element.

FIG. 1 is a block diagram of a conventional image display panel 1 of animage display device. The display panel 1 has a substrate 3 with asurface 5 which is provided with a pattern of pixels 7 which aremutually separated from one another in the vertical and horizontaldirections. Each pixel 7 comprises overlapping parts of electrodes 9which are provided in vertical columns, referred to as the columnelectrodes, and electrodes 11 which are provided in horizontal rows,referred to as row electrodes. In a plasma-addressed liquid crystaldisplay device (PALC), the rows are constituted by long, narrowchannels, referred to as compartments. The pixels 7 in each row ofelectrodes 11 represent one data row.

The width of the electrodes 9, 11 determines the dimensions of thepixels 7 which are typically rectangular. The electrodes 9 receive datadrive signals from a drive circuit 15 via parallel conductors 13, andthe electrodes 11 receive data drive signals from a drive circuit 19 viaparallel conductors 17.

To realize an image or a data-graphic reproduction on a relevant area ofthe surface 5 of the substrate 3, the display device uses a controlcircuit 21 which controls the drive circuits 15 and 19. Various types ofelectro-optical materials may be used in the display device. Examples ofsuitable electro-optical materials are twisted nematic or ferro-electricliquid crystalline materials. Generally, the electro-optical materialsattenuate the transmitted or reflected light, dependent on the voltagewhich is applied across the material.

FIG. 2 is a diagrammatic perspective view of a part of aplasma-addressed liquid crystal display device 23. The display device 23has a first substrate 25 and a second substrate 27. FIG. 2 shows onlythree column electrodes 29, 31, 33. The row electrodes 35, 37, 39, whichfunction as selection means, are constituted by a plurality of parallel,elongated channels under a layer of electro-optical material. An innersurface of each compartment is provided with first and second elongatedelectrodes 41, 43 which extend throughout the length of the channel. Thesecond electrode 43 is referred to as the anode and is fed with a pulsedvoltage referred to as strobe pulse, at which electrons emitted from thecathode 41 ionize the gas while forming a plasma. In an alternativeembodiment, the cathode is fed with a negative (DC) pulse. Only when thestrobe pulse has ended and the gas is deionized is the next channelswitched on. To reduce the cycle time, the next channel is usuallyalready ionized before the previous channel has been (completely)deionized. Since the column electrodes 29, 31, 33 each cross an entirecolumn of pixels, only one plasma row connection per unit of time ispossible so as to avoid crosstalk. The panel is provided with electricconnections to the column electrodes 29, 31, 33 and to the plasmaelectrodes 41, 43, the column electrodes 29, 31, 33 receiving drivesignals from output amplifiers 45, 47, 49, and the anode electrodes 43in the channels 35, 37, 39 receiving drive signals from outputamplifiers 51, 53. Each channel 35, 37, 39 is filled with an ionizablegas 55 and is sealed by a thin dielectric layer 57, referred to as themicrosheet, which consists of, for example glass.

The ionizable gas mixture comprises a basic gas to which an additionalgas is added. In such display devices, helium (He) is usually used as abasic gas for the ionizable gas mixture 55. Also nitrogen (N₂) or raregases such as, for example xenon (Xe), krypton (Kr) and neon (Ne) may beused as a basic gas. The ignition voltage of the plasma discharge may bedecreased by adding a given quantity of an additional gas (typically ofthe order of 0.1 to 3%) to the basic gas. A known gas which is added tothe basic gas so as to form Penning mixtures is hydrogen (H₂). Theproperties of the plasma discharge are influenced by using such gasmixtures. Instead of hydrogen, it is alternatively possible to adddeuterium or hydrogen deuterium, so that, in comparison with theaddition of hydrogen, the plasma discharge may have a lower ignition andsustain voltage and a longer afterglow decay time.

Due to the loss of additional gas, the partial pressure of this gas inthe mixture will not remain constant. To this end, the panel is providedwith means which are capable of supplying the additional gas andcompensate in this way for this loss and thus control the partialpressure of this gas in the gas mixture. Materials which are suitablefor this purpose, also referred to as guest materials, supply a constantpartial pressure of the additional gas within a given concentrationrange.

FIG. 3 shows diagrammatically the partial pressure of hydrogen as afunction of the hydrogen concentration in the guest material. Thecentral part of the curve has a plateau 60. In that part of the curve,the partial pressure of the additional gas is substantially independentof the concentration of the additional gas in the guest material.Moreover, it is desirable for the level of the plateau to be inconformity with the partial pressure of hydrogen desired in the displaydevice.

It is known that the addition of a given quantity of the above-mentionedadditional gases to a basic gas does not only influence the ignition andsustain voltage of the plasma discharge, but such additional gasesgenerally also have a detrimental influence on the afterglow decay timeT of the plasma discharge. The afterglow decay time is herein understoodto mean the time in which the conductivity of the plasma has decreasedby a factor of 1/e. The afterglow decay time is mainly determined by thepresence of metastable particles and their decay. Measurements haveproved that additions of H₂, D₂ and/or HD have the greatest influence onthe afterglow decay time of the plasma, which is caused by the Penningreactions at which metastable particles are quenched.

When the pressure of the ionizable gas mixture in a PALC display panelis approximately 20 kPa, the partial pressure p of the additional gas isapproximately 0.4 kPa. The formation heat ΔH of the desired material canbe calculated by means of the Van't Hof equation:${\ln \quad {p\left( H_{2} \right)}} = {\frac{\Delta \quad H}{RT} - \frac{\Delta \quad S}{R}}$

 ΔH=RT ln p(H₂)+TΔS

in which R is the gas constant (R=8.3 J/mol K), T is the temperature (inK) and ΔS is the entropy change for absorption of the additional gas(ΔS≈−130 J/mol K for H₂). At a temperature of T=293 K and p=0.4 kPa, avalue of the formation heat of ΔH≈−52 J/mol is thus obtained for H₂. Allhydride-forming materials having a plateau in the desired pressure rangeare suitable as means for dispensing H₂, D₂ or HD and/or for regulatingthe pressure of the H₂, D₂ or HD. Examples of suitable hydride-formingmaterials are VH₂, LH₃, PdH_(0.6), LaNi₅H₆, LaNi₂H_(x), LaCo₅H_(x),Zr-Mn-H_(x) or Pd-Ag-H_(x), in which LH₃ is a lanthanum hydride (forexample, LaH₃ or CeH₃) and in which one or more of the hydrogen atomsmay be replaced by deuterium. For example: VH₂ has a formation heatΔH≈−54 J/mol, which corresponds under the given circumstances to apartial pressure p(H₂) 0.15 kPa. PdH_(0.6) has a p(H₂)≈0.3 kPa. Theplateaus of LaNi₅H₆, LaNi₂H_(x) are above p(H₂)>0.1 kPa and, forLaCo₅H_(x), the partial hydrogen pressure under the given circumstancesis p(H₂)≈0.4 kPa. By forming compounds of, for example the typeLaNi_(5−x)Co_(x), the pressure in the display device can be adjusted atthe desired level.

In the known display devices, the means supplying deuterium, hydrogen orhydrogen deuterium are provided in, for example a space which is alsoreferred to as exhaust tube, or exhaust box, which comprises the exhaustconnection of the display device. The means may have the shape of, forexample a pellet, a wire, a foil or a powder. Since this exhaust tubecommunicates with the compartment or the compartments of the displaypanel, the partial pressure of the additional gas in each compartment iscontrolled. Instead of in the exhaust tube, the means may bealternatively provided in the compartment or the compartments. In thatcase, the guest material may form part of the electrodes, or theelectrodes may be provided with a layer of the guest material.

FIG. 4 shows a part of the display device of FIG. 2, showing only thesecond substrate 27. The first substrate, the microsheet, theelectro-optical layer, the column and row electrodes and the outputamplifiers, etc. have been omitted for the sake of clarity. Componentswhich are common to those in FIGS. 2 and 4 have the same referencenumerals in both Figures. In the example of FIG. 4, the channel plate 27is accommodated in a recess of a further substrate 59. The channel plate27 and the further substrate 59 may also constitute a solid part. Theionizable gas mixture 55 is present in the channels 35, 37, 39 of thechannel plate 27. The further substrate 59 has a recess or chute 61surrounding the channel plate 27, realizing a homogeneous gasdistribution in the display device. For filling the display device withthe desired ionizable gas mixture and for giving the display device thedesired pressure after the microsheet has been placed on the channelplate 27, the display device is provided with an exhaust box or exhausttube 63. The exhaust tube 63 is thus connected to the plate 64 of thepanel accommodating the compartments.

The means regulating the pressure of the additional gas, for examplehydrogen, deuterium or hydrogen deuterium may be provided, for examplein the recess or chute 61 in this case.

The present invention proposes several ways in which the temperature ofthe means supplying the additional gas can be stabilized so that thehydrogen pressure in the display panel remains substantially constant.In fact, for the material which is used as a guest material, the partialpressure of hydrogen is dependent on the temperature, and this isundesirable.

In a first possible implementation, it is proposed that the means forsupplying the additional gas are provided on a Peltier element which isintegrated in the exhaust tube 63. An embodiment is shown in FIG. 5. Theguest material, in this example in the form of a pellet, is present onthe first ceramic plate 67 of a Peltier element 69. A p-type material 73and an n-type material 75 are present between the first ceramic plate 67and a second ceramic plate 71. These materials are electricallyconnected in series and thermally in parallel. Dependent on the suppliedcurrent, the temperature of the guest material can be raised or loweredby means of a Peltier element, so that the partial pressure of theadditional gas in the display panel can be maintained constant. Theouter side of the Peltier element 65, in other words the heat exchanger,is part of the outer side of the exhaust tube 63 so that heat can besupplied to the ambience or to a heat sink (not shown).

In a second possible implementation, it is proposed that a guestmaterial is provided on a wire, for example a metal wire which isbrought to a suitable temperature. Since the resistance of the wire isdependent on the temperature, the temperature can be measured andcontrolled by measuring the resistance. The wire is present in, forexample the exhaust tube 63. An embodiment is shown in FIG. 6. Thehelically shaped wire 77 is present in the exhaust tube 63 and isconnected to a supply unit which is combined with a thermometer. Thetemperature measurement may be effected, for example by means of ameasurement of the electrical resistance of the wire.

The temperature-stabilizing means do not need to be present within thepanel but may be alternatively present outside the panel in the displaydevice. The means may comprise, for example the background illuminationsystem of the display device. The background illumination system 50 isshown in the embodiment of the display device of FIG. 2. The displaypanel is illuminated substantially throughout its surface. Theillumination system 50 may comprise a meander-shaped lamp but mayalternatively comprise a number of separate lamps. The illuminationsystem supplies such a heat that, in combination with the suitablechoice as a guest material, the desired partial pressure of theadditional gas can be realized. A suitable material is understood tomean a material which has a relatively flat plateau of partial pressureof the additional gas within a given concentration range of additionalgas in the guest material. This embodiment is notably suitable in thosecases where the guest material is present in the compartments of thedisplay panel or in the recess 61 (FIG. 4).

Another possibility of temperature stabilization in the case where theguest material is present in the exhaust tube is to provide the exhausttube 63 externally with a heating element 81. The heating element maybe, for example a helical heating wire which can be brought to a giventemperature by means of a current source 83. An embodiment is shown inFIG. 7.

Still another possibility of temperature stabilization consists ofproviding the means for supplying the additional gas internally with aheating element. This can be achieved by using a pellet-like, additionalgas supplying means through which a wire extends, said wire having avariable temperature.

What is claimed is:
 1. A display device having a display panelcomprising at least one compartment which contains an ionizable gasmixture including at least a basic gas and an additional gas, thecompartment being provided with electrodes for selectively ionizing theionizable gas mixture during operation, and the display panel comprisingmeans for supplying the additional gas to the ionizable gas mixture,characterized in that the display device is provided withtemperature-stabilizing means for stabilizing the temperature of themeans for supplying the additional gas.
 2. A display device as claimedin claim 1, characterized in that the additional gas is a gas of thegroup constituted by hydrogen, deuterium, or hydrogen deuterium.
 3. Adisplay device as claimed in claim 1, characterized in that thetemperature-stabilizing means comprise a Peltier element accommodatingthe means for supplying the additional gas.
 4. A display device asclaimed in claim 3, comprising an exhaust tube, characterized in thatthe Peltier element is accommodated in the exhaust tube and in that atleast a part of the heat exchanger of the Peltier element forms part ofthe wall of the exhaust tube.
 5. A display device as claimed in claim 1,characterized in that the temperature-stabilizing means comprise a wireon which the means for supplying the additional gas are provided, thetemperature of said wire being variable.
 6. A display device as claimedin claim 5, comprising an exhaust tube, characterized in that the wireis accommodated in the exhaust tube.
 7. A display device as claimed inclaim 2, characterized in that the means for supplying the additionalgas comprise a material of the group constituted by VZ₂, LZ₃, PdZ_(0.6),LaNi₅Z₆, LaNi₂Z_(x), LaCo₅Z_(x), Zr-Mn-Z_(x) and Pd-Ag-Z_(x), in which Lis a lanthanide and Z is hydrogen and/or deuterium.
 8. A display deviceas claimed in claim 1, comprising a background illumination unit,characterized in that the background illumination unit forms part of thetemperature-stabilizing means.
 9. A display device as claimed in claim1, comprising an exhaust tube, characterized in that a heating elementsurrounds the exhaust tube.
 10. A display device as claimed in claim 1,characterized in that the means for supplying the additional gas areinternally provided with the temperature-stabilizing means.
 11. Adisplay device as claimed in claim 10, characterized in that the meansfor supplying the additional gas is pellet-like and thetemperature-stabilizing means comprise a wire which extends through thepellet-like means for heating these means.